Sundry calender machines are known in the art which utilize a plurality of rollers for applying compressive pressure to a workpiece. Particularly well known calender machines are apparatus for laminating a transparent plastic film to one or both sides of a substrate, e.g., a printed document, photograph, poster, etc., for the purpose of protecting the substrate from aging and environmental factors. Many laminating films are heat-activated. Accordingly, laminating machines also include a means of heating the film.
There are two general types of laminating machines. The first type heats the plastic film before it passes through a pair of pressure rollers. The substrate, or workpiece, and the heated plastic film are then pressed together at a nip point of the pair of pressure rollers. The second type of laminating apparatus heats one or both of the pressure rollers. The plastic film and substrate are fed into the nip of the heated rollers and heated precisely at the point of pressurization.
Both types of laminating machines usually include a second pair of rollers for moving the workpiece through the laminating process in a regular and orderly manner. This second pair of rollers, called the pull-rollers, are usually not heated. The pull-roller assembly includes a drive system for actively rotating one or both of the pull-rollers thereby pulling the substrate through the laminating machine. In this manner, the pull-rollers apply tension to the film. Moreover, compressive pressure is applied to the workpiece as it passes through the nip between the pull-rollers.
A particular problem associated with laminating machines is that heat must be applied evenly to the plastic film to ensure proper lamination. In addition, it is necessary to provide sufficient and uniform tension to the plastic film to ensure that the plastic film lies completely flat against the substrate.
When a thin substrate is laminated, e.g., paper, the finished workpiece has a tendency to curl. The curling problem tends to be more severe when the laminating film is pre-cut to the approximate size of the article to be laminated. In laminating machines that employ continuous strips or webs of laminating film, the curling problems can be reduced by winding the finished product around a collecting roll. Sometimes, a heating unit is interposed in advance of the collecting roll to re-soften the adhesive of the plastic film. However, the addition of a collecting roll and heating unit increases the cost of the equipment and is unsuitable for desk top and other small laminating machines.
Another problem with calender machines in general, is that a workpiece must be manually fed into each nip between the rollers. As can be expected in these situations, the operator, concentrating on ensuring a properly laminated substrate, occasionally has his or her fingers pinched in a nip.